Campaigners have objected to the route of the new pylons

It said the 400kv line from Beauly, near Inverness, to Denny, near Falkirk, was the "most extensive development" on which it had ever taken a view.

SNH said scrutiny of the environmental statement would take several months.

Power firm Scottish and Southern Energy said the upgraded line would ship renewable energy to the south.

Existing line

The current 132kv line would not be sufficient to feed power from the growing number of wind and hydro energy sites in the Highlands and islands to the National Grid, the company said.

After 18 months of consultation SSE has submitted a planning application to the Scottish Executive.

It proposes a 137 mile (220km) route which has about 600 pylons, some up to 213ft (65m) high.

The company said that overall there would be 200 fewer pylons than on the existing transmission line, but that they would be bigger.

Part of the proposed line will run through the Cairngorms National Park, which is one of the areas where the route has sparked protest.

Ramblers Association in Scotland president Cameron McNeish said it would spoil a remote and environmentally sensitive area.

It is seen as the key to unlocking the environmental and economic benefits of renewable energy production in areas which hold some of the best resources in Europe John Thomson SNH

SNH said that proposed route also crossed several internationally important sites for nature conservation.

It said the route had the potential to affect a number of species and habitats, ranging from lampreys to geese and woodland to bogs.

The natural heritage body will also consider how far the development was likely to impinge upon access and people's enjoyment of the countryside.

The SNH director with lead responsibility for renewable energy issues, John Thomson, said: "A project of this type and on this scale inevitably presents challenges for the developer and SNH alike.

"It is seen as the key to unlocking the environmental and economic benefits of renewable energy production in areas which hold some of the best resources in Europe.

"But it will also inevitably impact upon areas valued highly for their wildlife, habitats and landscapes, and for the outdoor recreation opportunities that they offer."